Towards a Mechanistic Understanding of the Tumor Suppressor Function of Wiskott-Aldrich Syndrome Protein

Abstract

Wiskott-Aldrich syndrome (WAS) is a rare pediatric disorder caused by mutations in the WAS gene. The biological features of this disease include thrombocytopenia, eczema, complex immunodeficiency, and malignancy. WAS protein (WASP), encoded by the WAS gene, is a classical actin nucleation-promoting factor. Yet, the well-known functions of WASP fail to fully explain the high rate (13%~ 22%) of cancer in children with WAS. Recently, WASP was identified as a tumor suppressor by Chiarle’s group; however, the mechanism of its tumor suppressor function is not clear. Mounting evidence has already demonstrated that the ribosomal DNA (rDNA) gene inside the nucleolus is critical for genome stability, chromatin structure, and cancer pathogenesis. In addition, the perinucleolar heterochromatin shows structural alterations in cancer cells. Here, we use induced pluripotent stem cells (iPSCs) from patients with WAS (WAS-iPSC), isogenic gene- corrected cells (cWAS-iPSC), WASP knock out iPSCs, and B lymphoblastoid cell lines to study the mechanisms of WAS pathogenesis. Our results show that WASP interacts physically with partners inside the nucleolus and binds to the rDNA. Mutation cells undergo 5S rDNA copy number amplification and 45S rDNA array loss. WASP deficiency results in perinucleolar heterochromatin lost, irregulate nucleolus shape, and chromosomal aberrations. Taken together, our results show that WASP is important for genome stability, revealing its tumor suppressor mechanisms in blood cells.